The present invention pertains to high temperature, rotary calciners retrofitted from natural gas- or oil-firing to the use of coal as fuel and to their method of operation.
Rotary calciners, which are also called rotary kilns or rotary furnaces, are employed for calcining or roasting a variety of solid materials at high temperatures. U.S. Pat. Nos. 3,131,996, 3,838,189 and 3,395,906 disclose the use of rotary calciners in soda ash processes for converting trona ore (crude sodium sesquicarbonate) to a crude anhydrous sodium carbonate. U.S. Pat. No. 3,813,210 describes the use of rotary kilns for producing cement clinker from limestone and clay. Rotary kilns are also widely used for calcining limestone (calcium carbonate) to lime (calcium oxide).
Clean-burning fuels like natural gas or fuel oil are generally used to fire such high temperature calciners or kilns, particularly since these fuels readily provide the high temperature combustion off-gas desired for calcination.
Because of limited availability and rising costs of natural gas and fuel oil, it is becoming increasingly desirable to convert rotary calciners fired by these fuels to coal, a cheaper, more plentiful alternative fuel.
Conversion, or retrofit, of a rotary calciner to coal normally results in a significant reduction in the solids-processing capacity of the kiln, since the coal combustion gas temperature must be held below the softening temperature of the coal ash. This maximum allowable gas temperature for coal combustion is usually much less than the combustion gas temperatures achieved with natural gas or oil as a fuel. Consequently, a significantly larger volume of coal combustion gas must be employed to supply the calcination heat input required for a given amount of solid material, assuming that the low gas temperatures provide sufficient driving force to effect calcination.
One way in which solids-processing capacity in a coal-retrofitted calciner can be maintained at the original design level is to provide additional heat by preheating the solid material before it is introduced to the calciner. This is often an unsatisfactory solution since the preheating apparatus entails a substantial capital investment.
Another approach to this dilemma is to modify the interior of the calciner to increase the residence time of the solids, despite higher gas flows. This solution also can involve major, costly modifications to the calciner and is often not a practical answer because of the high gas flows, i.e., gas velocities, required to supply the needed heat input. Furthermore, despite the presence of sufficient heat input, the lower temperature of the coal combustion gas sometimes precludes proper calcination of the solid being processed.
The present invention provides a relatively simple solution for achieving a coal-retrofit of a natural gas- or oil-fired calciner, without the drawbacks of low combustion gas temperature or high retrofit equipment costs.